The present invention relates in general to radio receivers with reduced distortion during multipath events, and more specifically to fast detection of a multipath event in an FM stereo receiver.
The problem of multipath distortion in radio receivers is well known. Multipath occurs when radio frequency (RF) signals following direct and indirect (i.e. reflected) paths from a transmitter to a receiver interfere with each other at the receiver. Reflections can be caused by hills and buildings, for example.
Constructive and destructive interference of signals caused by interaction between the reflections and the direct line of sight transmission causes both signal distortion and rapid fluctuations in the received field intensity, especially in moving vehicles.
Multipath is a particularly annoying problem in reception of FM stereo broadcasts. A standard FM stereo signal includes a 19 kHz pilot carrier which is transmitted for the purpose of recovering the L-R frequency-multiplexed stereo signal. When a multipath event occurs, the FM carrier signal experiences an impulse phase shift. The phase shifted FM signal is applied to the input of an RF mixer in the typical superheterodyne receiver. Due to the nonlinear characteristics of the mixer, the phase distortion is intensified and the duration of the phase disturbance is lengthened. The phase disturbance continues to lengthen in each succeeding section of the receiver. Demodulation of the FM signal produces a phase-shifted pilot signal which is then applied to the stereo decoder. The phase shift causes an abrupt unlocking of the phase-locked loop (PLL) normally employed in the stereo decoder to regenerate the pilot carrier. A rasping sound is heard in the audio output when the PLL is violently pulled out of lock by the multipath interference.
Prior art radio receivers are known wherein stereo separation is decreased during a multipath event in order to reduce the objectionable sounds associated with multipath. However, stereo separation cannot be changed fast enough to suppress all the multipath distortion. Furthermore, changing the stereo separation is itself a type of undesirable distortion. In addition, changing the stereo separation fails to correct for the lengthening of the multipath disturbance in each section of the receiver.
There are several prior art methods for detecting the presence of multipath interference in a received broadcast signal. For example, it is known to detect multipath by measuring fluctuations in the received signal strength of the IF signal. This has been done by AM detection of a voltage signal which is proportional to received signal strength. Alternatively, multipath events can be detected by measuring particular noise components in the demodulated FM signal. However, these methods detect multipath only after the multipath has been present for an undesirably long time, allowing much distortion to reach the audible output.